1. Field of the Invention
The present invention relates to a stepping motor, and more particularly to a stepping motor which has a plurality of protrusions formed on a metal housing thereof, thereby attracting a permanent magnet thereof and locating it insitu or in a position of alignment when the stepping motor malfunctions.
2. Description of Related Art
A popular type of billboard includes a series of triangular bars which are simultaneously rotated to present three complete images when the bars complete one revolution. That is, a complete image is divided into strips corresponding to the quantity of triangular bars and adhered thereto. This process is repeated for the second and third images. The triangular bars are positioned such that one complete image is shown at one time, after which the triangular bars are partially rotated to display the second complete image and so on. A series of stepping-motors is used to drive the rotation of the triangular bars but if there is malfunction in the stepping-motors, the essential alignment will be lost, possibly rendering the advertising effect meaningless. A conventional stepping motor generally includes a shaft and an armature which is permanently magnetized and securely mounted around the shaft. At least one coil is used in the stepping motor to activate magnetic field. Typically, the coil is wound in a plane which intersects with the rotor disk. The stepping motor further has a metal housing enclosing the shaft, the rotor disk and the coil. When current continuously passes through the coil, the rotor disk will be driven to rotate, enabling the triangular rod connected therewith to locate in a proper position. A comprehensive description of a stepping motor in this art can be found in U.S. Pat. No. 4,831,293, which is incorporated herein by reference.
Although this conventional stepping motor has a small volume and a low cost when compared with LED-type billboards, there is a potential disadvantage in that the rotor disk may shift and cause the triangular rod to move to an improper position when a power failure occurs.
The present invention provides an improved stepping motor to mitigate and/or obviate the aforementioned problems.